Mozilla has released the first Thunderbird 3.1 beta for Windows, Mac and Linux.
Codenamed Lanikai, version 3.1 of Mozilla's open source email and news client is built atop Gecko 1.9.2, the same rendering engine at the heart of the new Firefox 3.6.
According to a Mozilla blog post, the first "stable" Lanikai release includes …

Re: Shared Contacts

The hard part with shared contacts, is where to put them. There are a number of Calandar protocols now (CalDAV, iCal, etc.), but nothing popular for address books. Funambol and SyncML are options, but you are unlikely to have a server that supports either. So how can Thunderbird implement shared contacts without this?

There are a number of Thunderbird plugins, which sync your Thunderbird to a proprietary mail systems. Zindus supports syncing your address book with Gmail, for instance.

Exchange Killer ?

Thunderbird will access LDAP servers (according to the appropriate page in its Options), but I've never tried that so can't comment on it.

Thunderbird is an e-mail client, not an e-mail server, hence it could not be an 'Exchange Killer' but may be an 'Outlook Killer' in the right circumstances.

I've used Thunderbird/Lightning (2.0.23) on all my PCs to replace Outlook and am quite happy with it so far, but it does seem slow to draw pages (when switching from Mail view to Calendar view for example). However, this may be because I store the mail and calendars on my home network NAS box, which is not the fastest NAS box around.